In human inhabited environments, and in environments where other dangers such as explosion, fire or toxicity may occur, there is very often a requirement to test for gas contaminants which may create a health or safety hazard. In particular, there is an increasing demand for devices to monitor a specific-atmosphere--generally an enclosed volume--for toxic or flammable gases. Also, particularly where the atmosphere being monitored is inhabited by humans, there is a specific requirement for sensors having a rapid and reliable response to such contaminating gases as carbon monoxide, oxides of nitrogen, sulfur dioxide, hydrogen sulphide, carbon dioxide, hydrogen, phosphine, arsine, methanol, volatile hydrocarbons, and so on. Any such gas requires a specific sensor cell design, and in any given installation there may be several sensor cells of the present invention installed; each installed sensor cell having electrolytes and catalysts chosen for each of the sensor cells to be reactive to the presence of specific contaminating gases being tested for.
In some circumstances, the enclosed volume being monitored may be monitored only for one or two specific contaminating gases, which gases are the only likely gas contaminants to occur in such atmosphere being monitored. An example may be storage rooms where hazardous chemicals may be kept, or production facilities where hazardous materials are being released or are being used in the manufacture of other materials. In that case, the possible gas contaminants are known and specific cell systems may be designed accordingly.
To satisfy the requirement to be able to monitor for the presence of gas contaminants, it is not only necessary that sensor cells be provided that are capable of being economically produced and therefore readily purchased, it is also necessary that such sensor cells shall have a reasonably long active lifetime when installed in place. Moreover, particularly where it is necessary to monitor for toxic or flammable gases where there may be humans in the environment being monitored, or where there is a specific hazard, such sensors cells may be capable of detecting the presence of low concentrations of contaminant gases being tested for, so as to provide sufficient warning before the concentration of contaminant gas reaches dangerous levels.
The present invention provides gas sensor cells that have a catalytically active sensor electrode which is exposed to the atmosphere being monitored; where the sensor electrode is separated from a counter electrode which may also function as a reference electrode, or a counter electrode and a reference electrode, by a suitable ion conductive electrolyte. The nature of the electrolyte, and the manner in which it is retained in position, is discussed hereafter. It is important, however, that the sensor electrode must be sufficiently sensitive to low concentrations of gas contaminants in the atmosphere being tested, and examples of such electrodes are discussed hereafter.
The present invention provides a structure for the electrochemical gas sensor cells which is essentially of a sandwich-type construction, where the outer frame members secure the electrodes in place, and sandwich a third frame member between them in which all or at least a portion of the electrolyte is located. In the usual embodiments, the outer frame members are electrically conductive, so as to exhibit specific conductivity characteristics; and also so as to provide means for connecting external electrical measuring means (or, in yet another alternative embodiment, an external power source) directly to the sensor cell frame members. Moreover, the outer frame members of the electrochemical sensor cell of the present invention provide means for sealing the structure together, and thereby sealing means for the electrolyte chamber, as discussed hereafter. The third, intermediate, frame member is not electrically conductive, and it may be significantly thinner than the outer frame members.
Among the gases that may be tested for are gases and volatile substances as diverse as carbon monoxide, carbon dioxide, oxides of nitrogen, oxides of sulfur, hydrides of nitrogen such as ammonia and hydrazine, hydrides of phosphorus, sulfur, arsenic or boron, mercaptans, aldehydes, hydrogen, unsaturated and saturated hydrocarbon vapours, halocarbons and alcohols such as methanol. Indeed, in general a specific cell system can be devised using suitable catalysts and electrolytes to test for any toxic, combustible or flammable gas, or generally volatile substances which may be oxidizable. The enclosed volumes within which such gas contaminant monitoring may take place include those suggested above, and as well ordinary residential housing, parking garages of all sorts, vehicles, interiors of commercial or industrial buildings, hospitals, and mines.
Prior art devices have included various patented devices such as those described below; and in any event the prior art may generally be defined as comprising electrochemical sensors, ionization chamber sensors, photoelectric types of sensors, and metal oxide semiconductor devices. Most prior art sensors are solid stat or solid electrolyte, and may employ stabilized zirconia, yttria and tin oxides. However, any sensors that has heretofore been used for monitoring and/or controlling gas atmospheres has exhibited one or more of the following disadvantageous characteristics: (a) they most often have quite complex structures; (b) they very often must operate or can only operate at elevated temperatures (e.g. from 150.degree. C. to 600.degree. C.); (c) as well, or as a consequence of the above, they may require outside sources of electrical energy and/or heat to maintain their operating temperatures; (d) it follows that such devices may have long start-up or warm-up periods before reaching their operating characteristics; (e) moreover, nearly all prior art devices are costly to build and/or to operate; (f) and finally, the prior art devices are subject to deterioration over time, due to gas poisoning of their sensing systems and/or sensing elements.
Several specific prior art gas sensing elements or cells are as follows:
LaCONTI U.S. Pat. No. 4,025,412 describes a gas sensor which is capable of detecting oxidizable gases in air. The sensor cell is a laminated filter design, in which approximately ten components are used, placed on top of one another. The assembly is very expensive to produce, even in quantities, and the structure is susceptible to electrolyte leakage.
DEMPSEY et al, U.S. Pat. No. 4,227,984 provides a gas sensor having a solid polymer electrolyte, and being arranged so that a fixed potential difference between the reference and sensing electrodes is present at all times. The particular purpose of the Dempsey et al structure is to preclude the necessity for temperature compensation. DEMPSEY et al describe the use of noble metal and their alloys, or graphitic catalytic electrodes, whereby the catalyst particles are bonded to polymers such as PTFE. Thus, the electrodes comprise catalyst particles bonded to particles to a hydrophobic polymer, and as such they do not utilize a three dimensional sensing electrode active layer.
KITZELMANN et al, U.S. Pat. No. 4,394,239 teach an electrochemical sensor which is particularly adapted for detecting carbon monoxide. The electrochemical cell is enclosed in a plastic body, which serves only as a cell housing. Electrical connections are achieved by means of wires which are attached to the metal mesh embedded in the sensor and reference electrodes. Moreover, the acidic electrolyte is corrosive, thus expensive metals such as platinum have to be used as current collectors and electrical leads, resulting in increased costs. However, such metals are hydrophilic, which may result in electrolyte loss. KITZELMANN et al immobilize their electrolyte by forming a gel.
As to the KITZELMANN et al electrode, it is a sensing and counter electrode which contains platinum mesh as a current collector on the gas side. The platinum mesh contacts a carbon fleece, which has a layer of platinum black coated on it, and which contacts the immobilized electrolyte. The electrode-electrolyte-gas interface is a two dimensional zone which has an area substantially identical to the geometrical area of the electrode used. KITZELMANN et al make reference to conventional gas diffusion electrodes (col. 3, lines 1-8) which could require the sensing gas to be delivered into the sensing electrode under pressure, or which may be required to be rendered hydrophobic to avoid spilling of electrolyte.
VENKATASETTY, U.S. Pat. No. 4,522,690, in a structure intended particularly for sensing the presence of carbon monoxide, requires the use of a gelled aprotic organic non-aqueous electrolyte in which the contaminant gas to be detected is soluble together with platinum sensor and counter electrodes, and a silver reference electrode. VENKATASETTY utilizes a platinum film as the active part of the sensing electrode, and in the same manner as KITZELMANN et al, thus uses a two dimensional reaction-interface approach. Moreover, the VENKATASETTY structure is extremely expensive to produce, even in quantities.
HANDA et al, U.S. Pat. No. 4,543,273 teach a structure for sensing carbon monoxide, where a solid oxide electrolyte is used. The patent, however, relates particularly to the production of the sensing element, at very high temperatures (above 600.degree. C.). The sensor utilizes changes of conductivity of the sensing elements in the presence of gas contaminants so as to sense their concentration.
YAMAZOE et al, U.S. Pat. No. 4,718,992, describe a proton conductor gas sensor for the detection of hydrogen or carbon dioxide. In the sensing device, a pair of electrodes that are connected by a proton conductor are short circuited to cause protons to travel through the proton conductor. The difference in activity between the electrodes, or the difference in gas permeability between the electrode and the atmosphere, is used to detect various gas contaminants. The potential difference created within the proton conductor due to the proton movement is used as sensor output which is proportional to the concentration of the gas contaminant, and is relatively insensitive to humidity changes.
Co-pending application Ser. No. 07/513,441, referred to above, describes a novel sensor cell design where the cell is encapsulated. In that cell, conductive and non-conductive plastic materials are used from the sensor cell housing the plastic sensor cell is capable of making quantitative measurement of volatile gas contaminants at ambient temperatures below 100.degree. C.
In contradistinction to the above, the present invention provides sensitive electrochemical cells, and the electrodes for use therein, which will detect the presence of low concentrations of contaminating gases in air or other environments where they may be a possibility of dangerous or hazardous accumulations of such contaminating gases. These cells, and their electrodes, have the following advantageous characteristics: (a) the structure, and their production, are very simple; (b) they operate at various temperatures and do not require temperature control, nor do they require heaters to reach operating temperature; (c) they have a fast response time; (d) they have a high activity level, by which they may become active in the presence of very low concentrations of contaminating gas; (e) they are stable in their operation; (f) they have a long operating life; (g) and finally, they are inexpensive to produce.
The present invention provides an electrochemical gas sensor cell for quantitative measurement of gas contaminants in an atmosphere being monitored. In several of the preferred embodiments, the temperature of the gas atmosphere may be significantly higher than 100.degree. C. In any event, the gas sensor cells can be operated at temperatures up to the latent boiling or melting temperature of any of the sensor components being used. The gas sensor cell has a first sensor electrode mounted in a first frame member, a second counter electrode mounted in a second frame member, and a third frame member which contains an electrolyte chamber, where the first and second frame members are at opposite sides, respectively, of the third frame member which substantially contains the electrolyte. The manner in which the electrolyte is retained is such as to substantially accommodate changes in temperature or humidity of the atmosphere. Each of the first, second, and third frame members are formed of materials which are inert and impervious to the electrolyte. Those materials may be plastics, such as polypropylene, polyvinylidene difluoride, or acrylonitrile butadiene styrene (ABS), any of which may be filled with metals, carbon or graphite, or combinations thereof. Other suitable materials include ceramics which can be electrically non-conductive ceramics such as boron nitride, aluminum oxide, silica, or glass; or electrically conductive ceramics such as EBONEX.TM.--which is essentially Ti.sub.x O.sub.2x-1, where x=4 to 10 (usually x=4)--or other metal oxides, carbides, or sulfides, etc., may also be used.
At least the first sensor electrode, and optionally the second counter electrode is a porous electrode, with the first sensor electrode and optionally the second counter electrode having a catalyst dispersed thereon.
The sensor electrode is mounted in the first frame member so as to be exposed to the atmosphere being monitored, and the counter electrode is mounted in the second frame member so as to be isolated from any exposure to that atmosphere. Conductor means are associated with each of the first and second electrodes, and are connected to electrical measurement means and may also be optionally connected to additional electrical devices such as control or warning devices.
Thus, when the atmosphere being monitored contains a gas contaminant for which the catalyst on at least the sensor electrode and the electrolyte have been chosen to produce a change in the electrical characteristics of the sensor electrode with respect to the counter electrode in the presence of that gas contaminant, the electrical measurement means detects the change of electrical characteristic in such circumstances, the change of electrical characteristic being indicative of the presence of the gas contaminant being tested for. In that sense, the electrical characteristic may optionally provide a display of contaminant concentration, it may trigger a warning or control device, or it may be stored in a storage or memory device for later comparison and review. What that means is that electrical characteristic may be a potential which changes between the sensor electrode and the counter electrode, or the current flowing between those electrodes may change.
In an alternative embodiment, a reference electrode is located so as to be exposed to the electrolyte, and potentiometric measurement means are provided between the sensor electrode and the reference electrode, as well as the electrical measurement means being between the sensor electrode and the counter electrode.
The electrolyte may be immobilized by being absorbed in a matrix contained within the electrolyte chamber, or it may an ionically conductive solid polymer or ceramic electrolyte, or it may be a solid electrolyte capable of ionic motion within a solid lattice. Moreover, the electrolyte chamber may be in communication with an electrolyte reservoir which is formed within the third frame member, above the electrolyte chamber. Moreover, the electrolyte chamber may also extend into the first and second frame members.
Each of the first and second frame members may be electrically conductive plastics materials, or suitable polymer composites, or electrically conductive ceramics. The plastics materials of the frame members may be such as polypropylene, polyvinylidene difluoride, or acrylonitrile butadiene styrene (ABS), any of which may be filled with metal, carbon or graphite, or combinations thereof.
Other suitable stable and impervious polymers include polyvinyl chloride or acrylonitrile butadiene; suitable electrically conductive inert filler materials include titanium oxide, tungsten carbide and the like. Other inert conducting polymers may be such as those described in the Handbook of Conducting Plastics, Volumes 1 & 2, Marcel Dekker, 1986.
Certain suitable materials for use in the conductive frame member may be conductive ceramics such as EBONEX, (comprising Ti.sub.4 O.sub.7), other conductive titanium oxide compositions such as Ti.sub.5 O.sub.9 and TiO, other inert conductive metal oxides, i.e. Na.sub.2 WO.sub.3 carbides (e.g., WC, TiC, ZnC, Cr.sub.3 C.sub.2, etc.), sulfides (e.g., WS.sub.2, MoS.sub.2, etc.) borides and graphites, carbon and graphites, etc. as well as their mixtures such as TiC+C and doped materials.
In specific embodiments of the present invention, the second counter electrode may be exposed to a contained volume of cleaned or scrubbed air or other suitable gas which is substantially free of the gaseous contaminant being tested for. Moreover, in those circumstances the counter electrode is non-polarizable under ordinary operating conditions.
In a further embodiment of the present invention, the non-conducting polymer of the third frame member may have its surface plated with a suitable inert metal film such as gold, silver, platinum, or the like.
Potentiometric electric measurement means may be provided, whereby a change in the voltage developed between the sensor electrode and the counter electrode may be detected and measured. The electrical measurement means may also be amperiometric.
The catalyst, or the specific catalyst for each of the sensor electrode and optionally the counter electrode, is dispersed on the porous electrode substrate in such a manner as to have a high surface area. The catalyst of choice may be a high surface area noble metal supported or carbon, carbides, or other conductive substrates.
Still further, the present invention provides porous electrodes which may comprise at least one porous layer containing a catalytically active metal, alloy, or metal oxide--usually comprising at least one noble metal--carbon, and a polymeric hydrophobic binder. These electrodes may then be used in gas sensors according to the present invention, together with a stabilized liquid electrolyte or other suitable electrolyte; and are such that they may operate at ambient temperatures such as room temperature without the necessity for a prolonged start-up or warm-up period.
In the case of the electrolyte being a solid polymer or an ion conductive solid state material, an appropriate amount of electrolyte may be incorporated into the electrode during the fabrication process to facilitate "wetting" of the electrocatalyst. Alternatively, the finished electrode may be impregnated by the electrolyte in its liquid form--for example, DuPont NAFION.TM. dissolved in alcohol, or solid state salts in their molten form or dissolved in a suitable solvent. In any event, the objective is to create a three dimensional interface between the electrolyte, the electrocatalytically active sites, and the gas contaminant within the sensing electrode, so as to increase the sensitivity and reactivity of the sensing electrode based on the geometrical electrode surface area.
The present invention also provides methods for making the electrochemical gas sensor cell, where the assembly method may be such that the assembled frame members are exposed to heat at a temperature of less than about 400.degree. C. for a predetermined period of time, depending on the material being used. Indeed, particularly when plastic frame members are to be used, the methods of the present invention to provide the electrochemical gas sensor cell may be carried out at the relatively low processing temperature of about 165.degree. C., or they may carried out in an induction furnace; or when the surfaces of the frame members which face each other are coated with a titanium oxide powder, the assembly may take place using microwave technology.
Indeed, the electrochemical gas sensor cell of the present invention may be produced by injection moulding the exterior frame members around the other members of the cell which are already held in place within an injection mould.
In the case where at least one of the frame members is made entirely without the use of plastic materials--such as in the case of EBONEX conductive frame members--the sensor cell can be assembled by applying an inert adhesive such as a pressure sensitive adhesive onto the frame members to be joined. Then, the sensor cell can be assembled by merely applying slight pressure at room temperature to activate the pressure sensitive adhesive used. Of course other inert adhesives, epoxides, or hot melts can be utilized as well.
By all of the above, the present invention provides a low cost electrochemical gas sensor cell which may be arranged by the choice of suitable and specific catalysts and electrolytes to sense low concentrations of specific gases being tested for, and provide a rapid and reliable response in the presence of such gaseous or volatile contaminant.